1. Field of the Invention
The present invention relates generally to a pressure relief valve system that includes a pilot valve and, more particularly, to such a valve system in which the pilot valve has a radial damper mechanism that when used in conjunction with a pressurized liquid system, permits better control of the opening and closing characteristics of the closure elements in such valves.
Pressure lines, i.e., pressure vessels and piping systems, are frequently designed with pressure relief valves located at various locations therein to protect the pressure line from excessive pressure. These pressure relief valves are selfactuated devices that are set to open when the pressure in the line reaches a predetermined, maximum safe pressure level, typically referred to as the maximum allowable working pressure. When the pressure in the line reaches this maximum allowable working pressure, the closure element of the pressure relief valve moves away from the inlet nozzle and process fluid is allowed to flow out of the pressure line through the pressure relief valve. This fluid flow will continue at a rate sufficient to prevent the pressure in the pressure line from rising above a maximum allowable overpressure.
When the pressure in the pressure line falls below the set pressure of the pressure relief valve, the closure element in the pressure relief valve returns to the closed position, i.e., in contact with the nozzle, which prevents any additional flow from the pressure line.
Pressure relief valves are self-actuated, in that the forces required to open such valves are provided by the fluid at the pressure relief valve inlet. A particular problem exists when the fluid in the pressure line is an incompressible liquid due to the inertia of the incompressible fluid. Frequently, when dealing with incompressible liquids, the valve prematurely closes because the fluid in the pipeline is slow to move forward to hold the closure element open. As soon as the valve closes, however, the pressure in the pipeline again spikes back to the original pressure level thereby causing the valve to re-open. When this occurs, the pressure relief valve may become unstable as the closure element begins to cycle at high frequency which can cause damage to the pressure relief valve and to the system is being protected by the pressure relief valve.
2. Description of the Prior Art
Several methods have been employed to prevent a pressure relief valve from operating in an unstable manner when used in conjunction with incompressible liquid systems. One method is to design the valve to open in a proportional manner such that pressure is maintained at the valve inlet. An example of such a spring-loaded, pressure relief valve is described in U.S. Pat. No. 4,130,130 that issued to R. Stewart on Dec. 19, 1978. Similar technology has been applied to pilot operated pressure relief valves that employ a modulating pilot that controls the opening of the valve closure element in a proportional manner. Examples of such a valve is described in U.S. Pat. No. 4,586,533 which issued to R. Estes on May 6, 1986; U.S. Pat. No. 4,863,098 which issued to L. Kolze on Sep. 5, 1989; U.S. Pat. No. 5,058,624 which issued to L. Kolze on Oct. 22, 1991; and U.S. Pat. No. 5,950,657 which issued to Y. Lai on Sep. 14, 1999. These methods have improved the ability of pressure relief valves to operate in a stable manner when used in conjunction with an incompressible liquid system, however instability can and does still occur under a variety of system operating conditions.
Another approach is to introduce a damper into the pressure relief valve design which serves to reduce the severity of any oscillation and eliminate the oscillation once a steady state condition is achieved after the pressure relief valve has opened. Dampers may include viscous dampers or snubbers, e.g. needle valves or fixed orifices in pilot operated pressure relief valve control lines. An example of a valve having such a damper is described in U.S. Pat. No. 4,799,645 which issued to K. Kramer on Jan. 24, 1989. Dampers are, however, generally costly and difficult to maintain since they are prone to clogging.
The current invention incorporates a new damper concept that can be incorporated into the internal components of a pilot operated pressure relief valve. It serves to reduce cost, eliminate the need for additional special maintenance, and overcome the potential for clogging.